The long-term objectives of this study are to investigate the factors responsible for the progressive layering and differentiation of cell types during histogenesis of the retina. Our initial hypothesis is that these changes are associated with cell surface alterations which can be detected by morphological, biochemical or immunological techniques. We have chosen two approaches to this research. In the first, retinas of different ages will be examined in the electron microscope after freeze fracture replication in order to visualize intramembranous structures. In the second, we will attempt to separate the retina into fractions representing the different cell types found in the different layers of the tissue and to characterize their surfaces by specific labeling of surface molecules and by immunological techniques.